Conventionally, a resin particle having a protrusion is applied to extensive uses such as resin additives (an anti-blocking agent and a light diffusion agent), delustering agents, toner additives, powder paints, water-dispersed paints, decoration boards additives, artificial marble stone additives, cosmetics fillers, chromatographic column packings, and substrates of conductive microparticles. The resin particle used for these uses (particularly, the resin particle used as the resin additives and the substrates of the conductive microparticles) is desired to have a protrusion that is resistant to the detachment from the resin particle, and have a uniform protrusion size and protrusion distribution density.
That is, the size and the light diffusivity of the protrusion are closely related since the light scattering mainly occurs at the interface (surface) of the resin particles when the resin particle having a protrusion is blended in a resin film as the resin additive such as a light diffusion agent. Accordingly, the light scattering intensity decreases when the protrusion size and the protrusion distribution density becomes not uniform, and it results in the deterioration problem in light diffusivity. In the case where the resin particle having a protrusion is blended in a resin film as the resin additives such as an anti-blocking agent, the detachment of the protrusion by friction disadvantageously causes deterioration in blocking resistance, and the damage on the surrounding resin film due to the detached protrusion. Furthermore, in the case where the resin particle is applied to the substrate of the conductive microparticle, the detachment of the protrusion causes a problem that conductive connection stability provided by the protrusion is not be expected. Therefore, as the resin particle having a protrusion, it is desired to have a protrusion that is more resistant to the detachment, and have a uniform protrusion size and protrusion distribution density.
Therefore, resin particles having various protrusions are proposed. For example, Patent Document 1 proposes a resin particle obtained by subjecting a vinyl-based monomer and a non-crosslinking acrylic polymer to suspension polymerization in the presence of a unresponsive phosphate compound as a surfactant. Patent Document 2 proposes a concavo-convex particle obtained by chemically bonding a small-diameter particle on the surface of a large-diameter particle with the mutually reactive functional group applied on the surface of two kind of particles with different diameter. Furthermore, Patent Document 3 proposes a silicone microparticle obtained by producing granular polyorgano silsesquioxane via condensation of organotrialkoxysilane to adhere to the surface of a silicone elastomer spherical microparticle.
Conventionally, a connection method using an anisotropic conductive material is adopted in order to provide electric connection between a large number of facing electrodes or wirings in the assembly of an electronic device. The anisotropic conductive material is a material obtained by dispersing conductive microparticles in a binder resin. Examples thereof include an anisotropic conductive paste (ACP), an anisotropic conductive film (ACF), an anisotropic conductive ink, and an anisotropic conductive sheet. Used as the conductive microparticle for an anisotropic conductive material is conductive microparticle obtained by covering the surface of the resin particle as a substrate with a conductive metal layer as well as a metal particle. In the conductive microparticle comprising the resin particle and the conductive metal layer, electric connection between electrodes or wirings is provided by the metal layer having conductivity formed on the surface.
In this case, in order to secure sufficient conductive connection reliability, several attempts are made to form a protrusion on the conductive microparticle surface. For example, Patent Document 4 describes a method including simultaneously forming a minute protrusion when forming a nickel or nickel alloy coating on the surface of a spherical core particle with a electroless plating method. However, this method uses the abnormal precipitation of a plating and it is necessary to control an electroless plating condition to a special condition in order to form the minute protrusion, and therefore it becomes difficult to control the shape of the minute protrusion to a fixed range in some cases. Patent Document 5 describes a method to form a protrusion part by adsorbing a non-conductive inorganic particle to a plastic core and forming a metal plating layer, however, the protrusion part tends to be detached when the adsorption force of the plastic core and non-conductive inorganic particle is not adequate, and sufficient conductive connection reliability is not acquired in some cases.